1. Field of the Invention
The present invention relates to a display device. More particularly, the present invention relates to a liquid crystal display (LCD) device.
2. Description of Related Art
Recently, LCD device has gradually become the mainstream of display device because of their advantageous features of light weight, compact size, suitable for large or small area application, low operation voltage, low power consumption, and low radiation. Especially, LCD device is more applicable for portable electronic device such as the screen of notebook, mobile phone, or personal digital assistance (PDA). Therefore, the LCD device has become an indispensable device and its development is very important.
FIG. 1 is a schematic view of a conventional LCD panel system. As shown in FIG. 1, a conventional LCD panel system 100 generally comprises a LCD panel 102, a gate driver 104 and a source driver 106. The LCD panel 102 comprises a pixel array constructed by a plurality of pixels. For example, in a conventional LCD panel having resolution of 1024×768, the pixels are arranged in a matrix with 1024 columns and 768 rows, wherein each pixel comprises three sub-pixels having red, green and blue colors respectively. Therefore, the sub-pixels are arranged in a matrix with 3072 columns and 768 rows in the foregoing liquid crystal panel. As shown in FIG. 1, each pixel 112 in the first column of the LCD panel 102 comprises three sub-pixels, i.e., a red sub-pixel 112r, a green sub-pixel 112g, and a blue sub-pixel 112b. In addition, the first row also comprises other pixels such as pixel 114 and so on. Each sub-pixel comprises a thin film transistor (TFT) and a storage capacitor, wherein the storage capacitor is formed by a pixel electrode (not shown) connected to the drain of the TFT, a common electrode and a dielectric layer disposed therebetween. The gate of the TFT is controlled by the gate driver 104 via a corresponding scan line SL1, SL2 . . . or SLm. For example, the gates of the thin film transistors of the sub-pixels 112r, 112g and 112b is controlled by the scan line SL1. The source of the TFT is controlled by the source driver 106 via a corresponding data line DL1, DL2 . . . or DLn. For example, the sources of the thin film transistors of the sub-pixels 112r and 122r are controlled by the data line DL1.
The gate driver 104 receives a basic clock and a start pulse. After the start pulse is received by the gate driver 104, a plurality of scan signals are generated by the gate driver 104 according to the basic clock and output to the scan lines SL1, SL2 . . . and SLm sequentially.
The source driver 106 receives a digital input data in serial, and then the digital input data is converted into an analog data and output to data lines DL1, DL2 . . . and DLn in parallel simultaneously. Therefore, when the gate driver 104 receives the start pulse and output a scan signal to a specific scan line (e.g., scan line SL1) to turn on the gates of the thin film transistors of the pixels (e.g., the sub-pixels 112r, 112g, 112b etc.), the analog data is input to the sources of the thin film transistors of the sub-pixels 112r, 112g, 112b via the data lines DL1, DL2, . . . and DLn, and then the analog data is stored in the capacitor via the drain of the TFT.
After the source driver 106 receiving the digital input data, the digital input data is converted into the analog data via a digital to analog converter (DAC), wherein an applicable voltage is selected from a set of reference voltage and provided as the analog data according to the digital input data. For example, if the brightness of the digital input signal of the sub-pixel of the liquid crystal panel 102 as shown in FIG. 1 has 6 bits gray scale level, the set of reference voltage has 26=64 reference voltages. Thus, the brightness of the sub-pixel is dependent on the reference voltage stored in the storage capacitor thereof. In general, the relationship between the brightness BR, BG and BB of the three primary colors (red, green and blue) of the sub-pixels (e.g., sub-pixels 112r, 112g, 112b respectively) and the corresponding gray scale levels GR, GG and GB may be represent by the following equations (1-1) to (1-3):BR=GRγ  (1-1)BG=GGγ  (1-2)BB=GBγ  (1-3)γ represent gamma value parameter, conventionally, γ=2.2.
FIG. 2 illustrates relationships between the transmittance of the sub-pixels and the corresponding gray scale levels respectively corresponding to different color sub-pixels in a conventional LCD panel, wherein each sub-pixel includes a color filter to achieve the colorful displaying effect. It is noted that the property of liquid crystal (so called LC effect) may lead to variations among the transmittance of different color sub-pixels. Referring to FIG. 2, curve B1 represents the relationship between the transmittance and the corresponding gray scale level of the red sub-pixel (e.g., sub-pixel 112r); curve B2 represents the relationship between the transmittance and the corresponding gray scale level of the green sub-pixel (e.g., sub-pixel 112g); and curve B3 represents the relationship between the transmittance and the corresponding gray scale level of the blue sub-pixel (e.g., sub-pixel 112b). Specifically, corresponding to the same gray scale level, the transmittance of the blue sub-pixel is greater than that of the green sub-pixel, and the transmittance of the green sub-pixel is greater than that of the red sub-pixel due to the LC effect.
Besides, in order to reduce the pin count of the source driver 106, multiplexers are generally used to input the analog data to the data lines DL1, DL2, and DLn sequentially. FIG. 3 is a schematic circuit block diagram of one of the multiplexers. Referring to FIG. 3, the analog data AD from the digital to analog converter is input to the multiplexer 130. Then, switches SW1, SW2, and SW3 of the multiplexer 130 are turned on sequentially such that the analog data AD is input to the data lines DL1, DL2, and DL3 sequentially along a scan direction D. Since the analog data AD is input sequentially along the scan direction D, a coupling effect of voltage will generated when the sub-pixels 112r, 112g, 112b are driven via the data lines DL1, DL2, and DL3. In general, the coupling voltage ΔV between the data lines and the sub-pixels can be represented by the following equation (2):ΔV=(Cpd/Ctotal)*Vx  (2)Cpd represents the parasitic capacitance between a sub-pixel and the nearby data line, Ctotal represents the total capacitance, and Vx represents the applied voltage from the data lines. Accordingly, the actual voltage stored in the sub-pixels (e.g., sub-pixels 112r, 112g, 112b) in three primary colors (red, green and blue) can be respectively represented by the following equations (3-1) to (3-3):Vr=Vx+(2ΔV)  (3-1)Vg=Vx+(ΔV)  (3-2)Vb=Vx  (3-3)
In accordance with the equations (3-1) to (3-3), FIG. 4 is a plot of transmittance versus gray scale level of red, green, and blue sub-pixels with the coupling effect of voltage in a conventional LCD panel. Referring to FIG. 4, curve C1 represents the relationship between the transmittance and the gray scale of the red sub-pixel (e.g., sub-pixel 112r) with the coupling effect; curve C2 represents the relationship between the transmittance and the gray scale of the green sub-pixel (e.g., sub-pixel 112g) with the coupling effect; and curve C3 represents the relationship between the transmittance and the gray scale of the blue sub-pixel (e.g., sub-pixel 112b) with the coupling effect. It is noted that the coupling effect of voltage causes difference between the curves C1, C2, and C3, wherein the transmittance of the blue sub-pixel is greater than that of the green sub-pixel, and the transmittance of the green sub-pixel is greater than that of the red sub-pixel corresponding to the same gray scale level.
FIG. 5 is a plot of integration of the curves in FIG. 2 and FIG. 4 for illustrating actual transmittance versus gray scale level of red, green, and blue sub-pixels in a conventional LCD panel. Referring to FIG. 5, curve E1 represents the actual relationship between the transmittance and the gray scale of the red sub-pixel (e.g., sub-pixel 112r); curve E2 represents the actual relationship between the transmittance and the gray scale of the green sub-pixel (e.g., sub-pixel 112g); and curve E3 represents the actual relationship between the transmittance and the gray scale of the blue sub-pixel (e.g., sub-pixel 112b). Due to the integration of the LC effect and the coupling effect of voltage, the differences of transmittance between different color sub-pixels become more obvious. For example, the color of image tends to be blue, and the differences of transmittance affect the color fidelity of image.